Micro pick up array pivot mount with integrated strain sensing elements

ABSTRACT

Systems and methods for aligning a transfer head assembly with a substrate are disclosed. In an embodiment a pivot mount is used for generating a feedback signal in a closed-loop motion control system. In an embodiment, the pivot mount includes a plurality of spring arms, with each spring arm including a switch-back along an axial length of the spring arm such that a pair of first and second lengths of the spring arm are immediately adjacent the switch-back and are parallel to each other. A first strain sensing element is located at the first length, and a second strain sensing element is located at the second length.

BACKGROUND

1. Field

The present invention relates to micro devices. More particularlyembodiments relate to a micro pick up array pivot mount with integratedstrain sensing elements for aligning an electrostatic transfer headarray with a target substrate.

2. Background Information

The feasibility of commercializing miniature devices such as radiofrequency (RF) microelectromechanical systems (MEMS) microswitches,light-emitting diode (LED) display systems, and MEMS or quartz-basedoscillators is largely constrained by the difficulties and costsassociated with manufacturing those devices. Miniaturized devicemanufacturing processes typically include processes in whichminiaturized devices are transferred from one wafer to another. In onesuch implementation, a transfer wafer may pick up an array ofminiaturized devices from a donor wafer and bond the miniaturizeddevices to a receiving wafer. Methods and apparatuses for aligning twoflat surfaces in a parallel orientation have been described, and may beapplied to miniaturized device transfer.

SUMMARY

A pivot mount and transfer tool are described. In an embodiment a pivotmount includes a pivot platform, a base, and a plurality of spring arms.Each spring arm is fixed to the pivot platform at a corresponding innerroot, and fixed to the base at a corresponding outer root. Each springarm also includes one or more switch-backs along an axial length of thespring arm such that a pair of first and second lengths of the springarm immediately adjacent a switch-back are parallel to each other. Afirst strain sensing element may be located at the first length of thespring arm, and a second strain sensing element may be located at thesecond length of the spring arm. Likewise, a first reference gage may belocated adjacent the first strain sensing element at the first length,and a second reference gage may be located adjacent the second strainsensing element at the second length. For example, the strain sensingelements may be strain gages that are bonded to the spring arm,deposited on the spring arm, or doped regions in the spring arm. In anembodiment, the plurality of spring arms includes three or more springarms. In an embodiment, the one or more switch-backs includes an innerswitch-back along an inner length of a spring arm, and anouter-switchback along an outer length of the spring arm.

In an embodiment, the inner root is perpendicular to an inner length ofthe spring arm extending from the pivot platform, and the outer root isperpendicular to an outer length of the spring arm extending from thebase. In an embodiment, the pivot platform is movable relative to thebase in a direction orthogonal to a contact surface of the pivotplatform, and movement of the pivot platform in the direction orthogonalto the contact surface causes a normal strain at the surface of thespring arm that is characterized as being parallel to the axial lengthof the spring arm at the first and second lengths of the spring arm. Inan embodiment, the normal strain at the surface of the spring arm is ofopposite sign on the first and second lengths of the spring arm.

In an embodiment, a pivot mount includes a pivot platform with aplurality of compliant voltage contacts, a base, and a plurality ofspring arms in which each spring arm is fixed to the pivot platform at acorresponding inner root and fixed to the base at a corresponding outerroot. In an embodiment, each compliant voltage contact is at leastpartially formed by a channel extending through the pivot platform. Thecompliant voltage contacts may assume a variety of configurationsincluding a winding contour and switch-back. In an embodiment, the pivotmount includes a clamping electrode on the pivot platform. Eachcompliant voltage contact may protrude from the pivot platform such thatthey are raised above the pivot platform and clamping electrode.

In an embodiment, any of the pivot mounts described above may beincluded in a transfer tool, including an articulating transfer headassembly, and a micro mick up array mounted onto the pivot platform ofthe pivot mount. The micro pick up array may include a plurality ofelectrostatic transfer heads. In an embodiment, the pivot platformincludes a plurality of compliant voltage contacts as described above.The micro pick up array may include a plurality of voltage contactsarranged to mate with the plurality of compliant voltage contacts of thepivot platform. In an embodiment each electrostatic transfer head has alocalized contact point characterize by a maximum dimension of 1-100 μmin both the x- and y-dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is perspective view illustration of a mass transfer tool inaccordance with an embodiment.

FIG. 2 is a perspective view illustration of a micro pick up array andpivot mount mounted onto a transfer head assembly in accordance with anembodiment.

FIG. 3 is an exploded cross-sectional side view illustration of atransfer head assembly, pivot mount, and micro pick up array inaccordance with an embodiment.

FIG. 4 is a schematic top view illustration of a micro pick up array inaccordance with an embodiment.

FIGS. 5A-5E are cross-sectional side view illustrations of a method offorming a pivot mount including compliant voltage contacts in accordancewith an embodiment.

FIGS. 6-7 are top view illustrations of various structural features of apivot mount in accordance with an embodiment.

FIG. 8A is a top view illustration of a pivot mount including electricalrouting in accordance with an embodiment.

FIG. 8B is a close up view illustration of Detail A in FIG. 8A inaccordance with an embodiment.

FIG. 9A is an illustration of strain components in a body.

FIG. 9B is an illustration of strain components in a thin structure.

FIG. 10A is an illustration of a spring arm under pure bending inaccordance with an embodiment.

FIG. 10B is an illustration of a spring arm under simultaneous bendingand torsion in accordance with an embodiment.

FIG. 11A is a perspective view illustration of a pivot platform of pivotmount 300 deflected with a uniform z displacement in accordance with anembodiment.

FIG. 11B is a perspective view illustration of strain modeling fornormal strain in the x direction for a pivot platform deflected with auniform z displacement in accordance with an embodiment.

FIG. 11C is a perspective view illustration of strain modeling fornormal strain in the y direction for a pivot platform deflected with auniform z displacement in accordance with an embodiment.

FIG. 11D is a perspective view illustration of strain modeling forequivalent strain magnitude for a pivot platform deflected with auniform z displacement in accordance with an embodiment.

FIG. 11E is a perspective view illustration of strain modeling forsurface shear strain for a pivot platform deflected with a uniform zdisplacement in accordance with an embodiment.

FIG. 11F is a top view illustration of a pivot mount including eightcorrelated strain sensors in accordance with an embodiment.

FIG. 12 is top view illustration of a pivot mount in accordance with anembodiment.

FIG. 13A is a schematic illustration of a control scheme for regulatinga transfer head assembly in accordance with an embodiment.

FIG. 13B is a schematic illustration of a method of generating asynthesized output signal in accordance with an embodiment.

FIG. 13C is a schematic illustration of a method of generating asynthesized output signal in accordance with an embodiment.

FIGS. 13D-13E are schematic illustrations of methods of generatingsynthesized output signals in accordance with embodiments.

FIG. 14 is a flowchart illustrating a method of aligning a micro pick uparray relative to a target substrate in accordance with an embodiment.

FIG. 15 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION

Embodiments describe a pivot mount including a base, a pivot platform,and plurality of spring arms, each spring arm including a switch-backalong an axial length of the spring arm such that a pair of first andsecond lengths of the spring arm immediately adjacent the switch-backare parallel to each other. A first strain sensing element is located atthe first length, and a second strain sensing element is located at thesecond length of the spring arm. In this manner, when the pivot mount ismoved in a direction orthogonal to a contact surface of the pivotplatform normal strains of opposite sign are created at the surface ofthe spring arm on the first and second lengths of each spring arm.

The pivot mount can be coupled to an articulating head assembly of amass transfer tool for accurate and repeatable alignment in 6 spatialdegrees of freedom between the transfer tool and a target substrate.When accurately aligning two planar surfaces, lateral (x and y) androtational (θz) alignments are relatively straightforward to achievethrough use of a high-precision x-y stage and rotationally-positionedsubstrate chucks. The remaining three degrees of freedom, θx, θy, (or,“tilt” and “tip”) and z are difficult to independently control. Anychanges to the tip and tilt angle necessarily change the distance z toany point not located at the center of rotation. While parallelismbetween two planes can be accomplished through use of a passive pivotmount, the pressure distribution between the two planar surfaces willnot be centered or uniform unless the two surfaces were parallel tobegin with. A transfer tool including a pivot mount in accordance withembodiments described herein may redistribute the pressure distributionto achieve a uniform pressure field. By placing strain sensing elements(strain gages) at high-strain locations on the pivot mount spring arms,a feedback signal of the position error can be generated and input tothe transfer tool for operation in a closed-loop motion control system.Because strain is related to the stress state through Hooke's Law, bothdisplacement and forces acting on the pivot mount can be known bymeasuring strain.

In one aspect, embodiments describe a pivot mount configuration thatachieves a high strain sensing sensitivity and generates a feedbacksignal with a high signal to noise ratio. As a result the pivot mountcan provide a position feedback signal with increased effectiveresolution to the transfer tool. By locating the strain sensing elementson opposite sides of switch-backs in an axial length of a spring arm,equal and opposite strain responses are measured. In this manner astrain signal for a given platform displacement may be effectivelydoubled. Such a configuration can also reduce noise for a given strainsignal. Due to the differential sensing at the first and second lengthsadjacent the switch-back the measured noise is effectively canceled.Accordingly, higher strain sensing sensitivity may be accomplished witha higher signal to noise ratio, and an increased effective resolution ofthe position feedback signal may be provided to the transfer tool.

In another aspect, embodiments describe pivot mount spring armconfigurations that minimize the torsion applied to a spring arm at theroots where a spring arm is fixed to a pivot platform at one end andfixed to a base at another end. This creates a more uniform bendingmoment in the high strain regions of the spring arm with reduced strainvariation and torsion in the spring arms, which allows the strainsensing elements to be located in the high strain regions near theroots. By comparison, in other configurations with spring arms thatundergo both bending and torsional loading, the area of maximum strainmay include both bending and torsion. Torsion in the spring arms isparasitic to surface strain sensing since it manifests as strain at thesurface of the spring arm having components in both the x and ydirections. Because the total strain energy distributed through thespring arms is constant for a given pivot platform displacement, thepresence of strain components perpendicular to the strain sensingelements reduces the ratio of strain components that are aligned withthe strain sensing elements. As a result, strain sensing elementslocated near areas of torsion may produce a lower effective feedbacksignal and sensitivity. In an embodiment, a pivot mount is arranged tocreate boundary conditions at the roots of the spring arms with auniform bending moment, in which strain is substantially perpendicularto the roots and substantially parallel to strands in the strain sensingelements, which may be parallel to axial lengths of the spring arms inthe high strain regions. Such a configuration directs substantially allof the strain energy from a given pivot platform displacement intostrain components aligned with the strain sensing elements. As a result,higher strain may be measured and sense feedback signal strength may beincreased for a given pivot platform displacement. Reduction of thetorsional moment at the roots may additionally allow more freedom instiffness requirements of the spring arms. In turn, reduced stiffnessrequirements allow for longer axial length of the spring arms in thesame available real estate within the pivot mount, and consequentlygreater bending, resulting in increased normal strain at the surface ofthe spring arms and sense feedback signal strength.

In another aspect, reduction of the torsional moment applied to thespring arms at the roots may also increase the effectiveness of thereference gage(s) positioned adjacent the strain sensing elements. In anembodiment, each strain sensing element is located in a high strainregion of a spring arm that sees only normal strain at the surface inthe gage direction of the strain sensing element and sees no normalstrain at the surface lateral to the gage direction. This allows thelocation of a reference strain gage adjacent to each strain sensingelement with the result that the reference gages do not see straincaused by mechanical loading of the pivot platform. This in turn allowsthe reference gages to compensate for temperature variations in thesystem, and increase the signal to noise ratio. Since the strain sensingelements and reference gages are adjacent, they are exposed to the sametemperature, meaning the thermal strain is identical in both a strainsensing element and a corresponding reference gage. Since the referencestrain gages are not subjected to strain resulting from mechanical load,any strain signal they produce can be attributed to temperature (asnoise), which is then subtracted as background noise from the strainmeasured by the adjacent strain sensing element. In an embodiment,strands in the reference gages are oriented perpendicular to strands inthe strain sensing elements. In such a configuration, the normal strainat the surface of the spring arms is substantially parallel to thestrands in the strain sensing elements, and perpendicular to the strandsin the reference strain gages. Thus, by reducing the torsional momentand creating uniform bending moments in the spring arms in which normalstrain at the surface of the spring arms is substantially perpendicularto the roots, the reference strain gages may be more accurate and ahigher strain sensing sensitivity may be accomplished with a highersignal to noise ratio.

In another aspect, embodiments describe an arrangement of strain sensingelements into distributed, correlated sensors. In this manner, the lossof a strain sensing element or sensor does not prohibit use of the pivotmount, and the lifetime of the pivot mount use with a transfer tool canbe extended. In an embodiment, each sensor includes one or morecorrelation sensors. For example, a correlated pair may each sense asame z-deflection. In another situation, a correlated pair may sense asame or equal but opposite θx, θy, (or, “tilt” and “tip”). In eithersituation, the loss of one of the correlated sensor may reduce theoverall signal to noise ratio generated from the pivot platform, yet theremaining signal to noise ratio remains adequate for operation of thetransfer tool.

In yet another aspect, embodiments describe a pivot mount with compliantvoltage contacts, for providing a low contact resistance connections ofthe voltage contacts to a micro pick up array (MPA) that is mounted ontothe pivot platform of the pivot mount. The compliant voltage contactsmay protrude from the pivot platform such that they are elevated abovethe pivot platform, yet are compliant such that they exert a pressureupon the MPA contacts when the MPA is clamped onto the pivot mount pivotplatform, for example, using an electrostatic clamp contact on the pivotmount platform.

Referring to FIG. 1, a perspective view of a mass transfer tool isshown. Mass transfer tool 100 may include a transfer head assembly 200for picking up an array of micro devices from a carrier substrate heldby a carrier substrate holder 104 and for transferring and releasing thearray of micro devices onto a receiving substrate held by a receivingsubstrate holder 106. Embodiments of mass transfer tool 100 aredescribed in U.S. patent application Ser. No. 13/607,031, titled “MassTransfer Tool”, filed on Sep. 7, 2012. Operation of mass transfer tool100 and transfer head assembly 200 may be controlled at least in part bya computer 108. Computer 108 may control the operation of transfer headassembly 200 based on feedback signals received from various sensors(e.g. strain sensing elements, reference gages) located on a pivotmount. For example, transfer head assembly 200 may include an actuatorassembly for adjusting an associated MPA 103 with at least three degreesof freedom, e.g., tipping, tilting, and movement in a z direction, basedon feedback signals received from sensors associated with a pivot mountthat carries MPA 103. Similarly, the carrier substrate holder 104 andreceiving substrate holder 106 may be moved by an x-y stage 110 of masstransfer tool 100, having at least two degrees of freedom, e.g., alongorthogonal axes within a horizontal plane. Additional actuators may beprovided, e.g., between mass transfer tool 100 structural components andtransfer head assembly 200, carrier substrate holder 104, or receivingsubstrate holder 106, to provide movement in the x, y, or z directionfor one or more of those sub-assemblies. For example, a gantry 112 maysupport transfer head assembly 200 and move transfer head assembly 200along an upper beam, e.g., in a direction parallel to an axis of motionof x-y stage 110. Thus, an array of electrostatic transfer heads on MPA103, supported by transfer head assembly 200, and an array of microdevices supported by a carrier substrate held by carrier substrateholder 104 may be precisely moved relative to each other within allthree spatial dimensions.

Referring to FIG. 2, a perspective view of a transfer head assembly 200is shown in accordance with an embodiment. A transfer head assembly 200may be used in combination with mass transfer tool 100 to transfer microdevices to or from a substrate, e.g., receiving substrate or carriersubstrate, using MPA 103 which is supported by a pivot mount 300. Moreparticularly, transfer head assembly 200 may provide for negligiblelateral or vertical parasitic motion for small movements of MPA 103,e.g., motion less than about 5 mrad about a neutral position.Accordingly, transfer head assembly 200 may be incorporated in masstransfer tool 100 to adjust an MPA 103 relative to mass transfer tool100. Thus, transfer head assembly 200 may be fixed to a chassis of masstransfer tool 100, e.g., at a location along an upper beam or support.

As illustrated, the pivot mount 300 may include a base 302, a pivotplatform 304, and plurality of spring arms 306, and the MPA 103supporting a transfer head array 115 is mounted on the pivot platform304. In an embodiment, the transfer head array 115 is an electrostatictransfer head array 115, where each transfer head operates in accordancewith electrostatic principles to pick up and transfer a correspondingmicro device. In an embodiment each electrostatic transfer head has alocalized contact point characterized by a maximum dimension of 1-100 μmin both the x- and y-dimensions. In an embodiment, the pivot mount 300may communicate and send feedback signals to the mass transfer tool 100through one or more electrical connections, such as a flex circuit 308.As described below, feedback may include analog signals from strainsensing elements that are used in a control loop to regulate actuationand spatial orientation of the transfer head assembly 200. In anembodiment, the feedback signals are sent to a position sensing modulelocated near the pivot mount 300 to reduce signal degradation bylimiting a distance that analog signals must travel from a strainsensing element to the position sensing module. In an embodiment, theposition sensing module is located within the transfer head assembly200.

Referring now to FIG. 3, an exploded cross-sectional side viewillustration is provided of a transfer head assembly 200, pivot mount300, and MPA 103. Generally, the pivot mount 300 is mounted onto thetransfer head assembly 200. This may be accomplished using a variety ofmanners such as using tabs or lips to press the pivot mount against thetransfer head assembly 200, bonding, vacuum, or electrostatic clamping.A deflection cavity 202 may be formed in the transfer head assembly 200to allow a specified z-deflection distance of the pivot platform 200along the z-axis.

As illustrated in FIG. 3, the pivot mount 300 may include channels 310formed through a body of the pivot mount from a front surface 312 toback surface 314. Channels 310 may be used for form a variety ofcompliant features of the pivot mount 300, including defining the springarms 306 and pivot platform 304, as well as the compliant voltagecontacts 316, described in more detail in the following description. Thecompliant voltage contacts 316 may provide a low contact resistanceconnection to voltage contacts 120 of the MPA 103. In the embodimentillustrated, the compliant voltage contacts 316 protrude from the pivotplatform such that they are raised above the pivot platform. Uponclamping the MPA 103 onto the pivot platform of the pivot mount 300 withthe opposing electrostatic clamp contacts 318, 122, the compliantvoltage contacts 316 exert a pressure upon the MPA contacts 120.Additional features may be located on or in the pivot mount 300. Forexample, strain sensing elements 320 (strain gages) and reference gages340 may be located at high strain regions of the spring arms 306, asdescribed in further detail in the following description.

Referring now to FIG. 4, a schematic top view illustration of a MPA 103is shown in accordance with an embodiment. In an embodiment, an area ofthe electrostatic clamp contact 122 on a back side of the MPA is largerthan an area of the transfer head array 115 on the front surface of theMPA. In this manner, the alignment and planarity across the transferheads in the transfer head array 115 can be regulated by alignment ofthe transfer head assembly. In such an embodiment, a plurality ofvoltage contacts 120 for supplying an operating voltage to the transferhead array 115 is located outside the periphery of the transfer headarray 115.

Referring now to FIGS. 5A-5E, cross-sectional side view illustrationsare shown for a method of forming a pivot mount 300 including compliantvoltage contacts 316. The processing sequence may begin with acommercially available silicon wafer 301 including a top oxide layer330, and bottom oxide layer 332 as illustrated in FIG. 5A. While thefollowing description is made with regard to a silicon wafer,embodiments are not so limited, and other suitable substrates can beused to form pivot mount 300, such a silicon carbide, aluminum nitride,stainless steel, and aluminum, amongst others. In an embodimentillustrated in FIG. 5B, the top oxide layer 330 is then removed, withbottom oxide layer 330 remaining. Referring to FIG. 5C, the top andbottom surfaces of the wafer 301 may then be oxidized further resultingin a top oxide layer 334, and bottom oxide layer 336 that is thickerthan the previous bottom oxide layer 332 and thicker than the top oxidelayer 334. For example, this may be accomplished with a wet thermaloxidation operation. Following the formation of oxide layers 334, 336various layers may be formed over the top oxide layer 334 to form thestrain gages 320, reference gages 340, electrostatic clamp contact(s)318, and electrodes 317 for the compliant voltage contacts. In anembodiment, these various layers may be formed by one or more metaldeposition processes. In an embodiment, the electrodes 317 for thecompliant voltage contacts are thicker than other metallization layersused to form the strain gages 320, reference gages 340, andelectrostatic clamp contact(s) 318. Referring to FIG. 5E, the bottomoxide layer 336 is removed and channels 310 are etched through thesilicon wafer 301 and top oxide layer 334 to define the spring arms 306,pivot platform 304, and compliant voltage contacts 316. As shown in FIG.5E, the contact surfaces including the electrodes 317 for the compliantvoltage contacts 316 protrude from the pivot platform such that they areelevated above the surrounding pivot platform, including the straingages 320, reference gages 340, and electrostatic clamp contact(s) 318.This may be the result of releasing residual stress within the siliconwafer 301 during formation of the channels 310. In an embodiment, theresidual stress was created in the silicon wafer 301 during theoxidation and removal operation described and illustrated in FIGS.5A-5C. In accordance with embodiments of the invention, the channels 310forming the compliant voltage contacts 316 may assume a variety ofconfigurations such as switch-backs or a winding contour. In anembodiment, the channels forming the compliant voltage contacts 316 aremade in a spiral configuration which can achieve a high amount ofcompliance within a small area.

Referring again to FIG. 4, the voltage contacts 120 of the MPA 103 alignwith the compliant voltage contacts 316 in the pivot platform 304 of thepivot mount 300. Once the MPA is clamped onto the pivot mount pivotplatform, for example, using an electrostatic clamp contact on the pivotmount platform, the compliant voltage contacts 316 exert a pressure uponthe MPA voltage contacts 120 to achieve low contact resistanceconnections.

FIGS. 6-8B illustrate various structural aspects of a pivot mount 300.Referring to FIG. 6, in an embodiment pivot mount 300 includes a base302, a pivot platform 304, and a plurality of spring arms 306. Eachspring arm 306 is fixed to the pivot platform 304 at a correspondinginner root 350, and fixed to the base at a corresponding outer root 352.Each spring arm 306 includes at least one switch-back along an axiallength 354 of the spring arm such that a pair of lengths of the springarm adjacent the switch-back are parallel to each other. In theembodiment illustrated in FIG. 6, each spring arm 306 includes an innerswitch-back 356 along an inner length of the spring arm and an outerswitch-back 358 along an outer length of the spring arm. In anembodiment, an inner length 370 of the spring arm extending from thepivot platform 304 (along the axial length 354 of the spring arm 306) isperpendicular to the inner root 350. In an embodiment, an outer length372 of the spring arm extending from the base 302 (along the axiallength 354 of the spring arm 306) is perpendicular to the outer root352.

Referring now to FIG. 7, each switch-back along the axial length of thespring arm results in a parallel pair of lengths of the spring armadjacent the switch-back. For example, a portion of the spring armimmediately adjacent the outer switch-back 358 includes a first length360 and a second length 362 of the spring arm that are parallel to eachother. Similarly, a portion of the spring arm immediately adjacent theinner switch-back 356 includes a first length 364 and a second length366 of the spring arm that are parallel to each other. A first strainsensing element may be located at the first length of the spring armadjacent a switch-back, and a second strain sensing element may belocated at the second length of the spring arm adjacent the switch-back.Furthermore, first reference gage may be located adjacent the firststrain sensing element at the first length, and a second reference gagemay be located adjacent the second strain sensing element at the secondlength. In the particular embodiment illustrated in FIG. 7, a firststrain sensing element 320A is located at the first length of 360 thespring arm adjacent the outer switch-back 358, and a second strainsensing element 320B is located at the second length 362 of the springarm adjacent the outer switch-back 358. Furthermore, first referencegage 340A is located adjacent the first strain sensing element 320A atthe first length 360, and a second reference gage 340B is locatedadjacent the second strain sensing element 320B at the second length362. In the particular embodiment illustrated in FIG. 7, a first strainsensing element 320A is located at the first length of 364 the springarm adjacent the inner switch-back 356, and a second strain sensingelement 320B is located at the second length 366 of the spring armadjacent the inner switch-back 356. Furthermore, first reference gage340A is located adjacent the first strain sensing element 320A at thefirst length 364, and a second reference gage 340B is located adjacentthe second strain sensing element 320B at the second length 366.

Referring now to both FIGS. 6 and 7, in an embodiment the first andsecond lengths 364, 366 of the spring arm (along the axial length 354 ofthe spring arm 306) adjacent the inner switch-back 356 are perpendicularto the inner root 350. In an embodiment, the first and second lengths360, 362 of the spring arm (along the axial length 354 of the spring arm306) adjacent the outer switch-back 358 are perpendicular to the outerroot 352.

FIG. 8A is a top view illustration of a pivot mount including electricalrouting in accordance with an embodiment. As illustrated, wiring can berouted on the top surface of the pivot mount for operation of variouscomponents. In an embodiment wiring 380 is provided for operation of thestrain sensing elements 320 and reference gages 340. In an embodimentwiring 382 is provided for operation of the electrostatic clamp contacts318. In an embodiment wiring 384 is provided for operation of thecompliant voltage contacts 316. In the particular embodimentillustrated, the wiring 384 connects with the electrodes 317 for thecompliant voltage contacts 316, where the electrodes form a spiralpattern within the spiral channels 310 forming the compliant voltagecontacts. Wiring 380, 382, and 384 can run over one or more portions ofthe pivot mount including the base 302, spring arms 306, and pivotplatform 304. Wiring 380, 382, and 384 may be formed using a suitabletechnique such s sputtering or e-beam evaporation, or may be a wire thatis bonded to the pivot mount.

Wiring 380, 382, and 384 may be routed to an electrical connection, suchas a flex circuit 308, at an edge of the base 302 of the pivot mount.For example, an operating voltage can be applied trough the flex circuit308 to operate the electrostatic clamp contacts 318 to clamp the MPAonto the pivot mount 300. Another operating voltage can be appliedthrough the flex circuit 308 to operate the compliant voltage contacts316 which transfer an operational voltage to the array of electrostatictransfer heads in order to provide a grip pressure to pick up microdevices. Additionally, the flex circuit 308 can transfer the feedbacksignals from the strain sensing elements 320 and reference gages 340 toa position sensing module or computer 108 to regulate actuation andspatial orientation of the transfer head assembly 200.

Referring now to FIG. 8B, an enlarged view of Detail A from FIG. 8A isillustrated. In the particular embodiment illustrated the strain sensingelements 320 and reference gages 340 along the first and second lengths364, 366 of the spring arm adjacent the inner switch-back 356 are shownin more detail. In an embodiment, strain sensing elements 320 may bestrain gages that measure deformation of spring arm 306. The straingages may exhibit an electrical resistance that varies with materialdeformation. More specifically, the strain gages may deform when springarm 306 deforms. That is, the strain gage design may be selected basedon environmental and operating conditions associated with the transferof micro devices from a carrier substrate, to achieve the necessaryaccuracy, stability, cyclic endurance, etc. Accordingly, the straingages may be formed from various materials and integrated with thespring arm in numerous ways to achieve this goal. Several suchembodiments are described below.

A strain gage may be separately formed from spring arm 306 and attachedthereto. In an embodiment, the strain gage includes an insulativeflexible backing that supports a foil formed from polysilicon andelectrically insulates the foil from spring arm 306. The foil may bearranged in a serpentine pattern, for example. An example of anattachable strain gage is a Series 015DJ general purpose strain gagemanufactured by Vishay Precision Group headquartered in Malvern, Pa. Astrain gage that is separately formed from spring arm 306 may beattached to spring arm 306 using numerous processes. For example, thestrain gage backing may be directly attached to spring arm 306 with anadhesive or other bonding operation. More specifically, strain gagebacking may be fixed to a surface of spring arm 306 using solder, epoxy,or a combination of solder and a high-temperature epoxy.

In another embodiment, a strain gage may be formed on spring arm 306 ina desired pattern, such as a serpentine pattern. In an embodiment, astrain gage may be formed directly on spring arm 306 using a depositionprocess. For example, constantan copper-nickel traces may be sputtereddirectly on spring arm 306 in a serpentine pattern. The dimensions of astrand of a sputtered strain gage having a serpentine pattern may beabout 8 micron width with about an 8 micron distance between strandlengths and may be deposited to a thickness of about 105 nanometers.

In another embodiment, the material of spring arm 306 may be modified toform an integrated strain gage. More specifically, spring arm 306 may bedoped with a piezoresistive material to create a strain gage withinspring arm 306. As an example, the surface of spring arm 306 may bedoped silicon. The doped material may be in a serpentine pattern, havingdimensions that vary with an applied strain. Thus, the strain gage maybe fully integrated and physically indistinct from the remainder ofspring arm 306.

During the transfer of micro devices from a carrier substrate, springarm 306 and strain sensing elements 320 may be subjected to elevatedtemperatures, and thus, temperature compensation may be necessary. In anembodiment, strain sensing element 320 (strain gage) may beself-temperature compensated. More specifically, strain gage materialmay be chosen to limit temperature-induced apparent strain over theoperating conditions of the transfer process. However, in an alternativeembodiment, other manners for temperature compensation may be used. Forexample, temperature compensation may be achieved using a reference gagetechnique.

In an embodiment, strain sensing element 320 may be a strain gage onspring arm 306 having a pattern (e.g. serpentine) of lengthwise strandsthat align in a direction of anticipated normal strain at the surface ofthe spring arm. Still referring to FIG. 8B, in an embodiment, areference gage technique utilizes a reference gage 340 to compensate forstrain sensing element 320. More particularly, reference gage 340 may belocated adjacent strain sensing element 320 in the same area of strain.While strands of strain sensing element 320 may align with the directionof applied strain, strands of reference gage 340 may extendperpendicular to the strands of strain sensing element 320 and to thedirection of applied strain. Alternatively, reference gage 340 may belocated in a non-strain area of the pivot mount 300, apart from strainsensing element 320, which is located in a high strain area of springarm 306. For example, reference gage 340 may be located on base 302 orpivot platform 304. In each configuration, strain sensing element 320detects a strain applied to spring arm 306 and reference gage 340detects strain from thermal effects on the pivot mount 300. Accordingly,a comparison of strain in the strain sensing element 320 and referencegage 340 may be used to determine, and compensate for, strain related tothermal expansion of spring arm 306.

In particular, the strands 341 in the references gages 340 are orientedperpendicular to strands 321 in the strain sensing elements 320. As willbecome more apparent in the following description, the normal strain atthe surface that results at the first and second lengths 364, 366 of thespring arm during operation of the pivot mount is substantially parallelto the strands 321 in the strain sensing elements, and perpendicular tothe strands 341 in the reference strain gages. Similar strainrelationships are found at all of the inner switch-backs 356 and outerswitch-backs 358, wherein normal strain at the surface that occursduring operation of the pivot mount is substantially parallel to thestrands in the strain sensing elements 320.

Referring now to FIG. 9A, strain at any point in a body may be describedby nine strain components. These include three normal strains (εx, εy,εz) and six shear strain components (εxy, εxz, εyx, εyz, εzx, and εzy).Strain components in a thin structure are illustrated in FIG. 9B. For athin pivot mount structure shear strains on the surface (εzx and εzy)and out-of-plane normal strain (εz) are not significant. Thisidealization is known as plane stress. Accordingly, in an embodiment thestrain gages (strain sensing elements and reference gages) on thesurface of the pivot mount will measure components of the normal strainsεx and εy. In an embodiment, the pivot mount includes regions of strainloaded only in either pure εx or pure εy and directs substantially allavailable strain into measurable strain.

Referring now to FIGS. 10A-10B, the idealization of plane stress isillustrated as realized in accordance with embodiments. FIG. 10A is anillustration of a spring arm under pure bending in accordance with anembodiment. In such an embodiment, the spring arm undergoing purebending may have a single normal strain component aligned with thespring arm axial length. A reference gage 340 may be orientedperpendicular to the spring arm axial length and not measure any straindue to bending. FIG. 10B is an illustration of a spring arm in bothbending and torsion. In such a configuration, normal strain componentsand shear strain components are produced in multiple directions. In thiscase both the strain gage 320 and reference gage 340 may measurenon-zero strain, which may reduce the ability of the reference gage 340to compensate for temperature changes.

In order to illustrate strain confinement within the pivot mount, apivot mount with a uniform z displacement of the pivot platform 304 isillustrated in FIGS. 11A-11E along with modeling data for strain fieldslocated within the pivot mount. Referring to FIG. 11A, a pivot platform304 of pivot mount 300 is deflected with a uniform z displacement. Suchdeflection may be typical during a normal pick and place operation withthe mass transfer tool, though the amount of deformation illustrated inFIG. 11A is exaggerated for illustrational purposes. In the particularembodiment illustrated in FIG. 11A, the spring arm 306 along the firstlength 360 the spring arm adjacent the outer switch-back 358 and thefirst length 364 of the spring arm adjacent the inner switch-back 356has a negative curvature and is in a condition of negative (compressive)normal strain at the surface. In the particular embodiment illustratedin FIG. 11A, the spring arm 306 along the second length 362 the springarm adjacent the outer switch-back 358 and the second length 366 of thespring arm adjacent the inner switch-back 356 has a positive curvatureand is in a condition of positive (tensile) normal strain at thesurface.

In accordance with embodiments of the invention, a pivot mount structureachieves a high strain sensing sensitivity and generates a feedbacksignal with a high signal to noise ratio by locating strain sensingelements on opposite sides of switch-backs in an axial length of aspring arm, where equal and opposite strain responses are measured. Inthis manner, strain signal for a given platform displacement may beeffectively doubled, while also reducing noise for a given strain signalsince the differential sensing can be used to effectively cancel thenoise.

Referring to FIG. 11B, modeling data is provided for the z displacementillustrated in FIG. 11A illustrating normal strain at the outer surfaceof the pivot mount in the x direction, εx. As illustrated, each springarm 306 includes an outer switch-back 358 oriented in the y-direction,and an inner switch-back oriented in the x-direction. Of course, simplyrotating the pivot mount reverses the orientations of the switch-backsin the x- and y-directions. Importantly, when in the condition ofuniform z displacement, the high εx strain regions are located along thespring arms adjacent the inner switch-backs 356, while minimal or no εxstrain is located along the spring arms adjacent the outer switch-backs358. Some amount of localized strains are found at various locationswithin the pivot mount due to local stress concentrations, however thesedo not affect the strain measurement because the strain gages arelocated away from the localized strain regions 365. As shown in the FIG.11B, the spring arm 306 along the first length 364 of the spring armadjacent the inner switch-back 356 has a negative curvature and is in acondition of negative (compressive) normal strain at the surface, andthe second length 366 of the spring arm adjacent the inner switch-back356 has a positive curvature and is in a condition of positive (tensile)normal strain at the surface. Furthermore, the negative normal strain atfirst length 364 and positive normal strain at second length 366 areequal and opposite.

Referring to FIG. 11C, modeling data is provided for the z displacementillustrated in FIG. 11A illustrating normal strain at the outer surfaceof the pivot mount in the y direction, εy. When in the condition ofuniform z displacement, the high εy strain regions are located along thespring arms adjacent the outer switch-backs 358, while minimal or no εystrain is located along the spring arms adjacent the inner switch-backs356. Some amount of localized strains are found at various locationswithin the pivot mount due to local stress concentrations, however thesedo not affect the strain measurement because the strain gages arelocated away from the localized strain regions 367. As shown in the FIG.11C, the spring arm 306 along the first length 360 of the spring armadjacent the outer switch-back 358 has a negative curvature and is in acondition of negative (compressive) normal strain, and the second length362 of the spring arm adjacent the outer switch-back 358 has a positivecurvature and is in a condition of positive (tensile) normal strain.Furthermore, the negative normal strain at first length 360 and positivenormal strain at second length 362 are equal and opposite.

FIG. 11D is an illustration of modeling data for equivalent strainmagnitude at the outer surface of the pivot mount in both εx and εy forthe z displacement illustrated in FIG. 11A. As shown, substantiallyequal strain magnitudes are measured at the first and second lengths forboth the inner and outer switchbacks. FIG. 11E is an illustration ofmodeling data for shear strain at the outer surface of the pivot mountfor the z displacement illustrated in FIG. 11A. As illustrated, there issubstantially no measurable shear strain at the surface. Thus, themodeling data provided in FIGS. 11A-11E illustrates a pivot mountconfiguration with substantially uniform bending moments in the highstrain regions of the spring arms.

Strain sensing elements 320 and reference gages 340 may be arranged intosensors so that the resulting sensor signals are correlated. A set ofsensors is considered correlated, or dependent, if the signal of amissing or broken gage in the sensor may be approximated from theremaining set of signals. A minimum set of independent strain signalsequal to the number of desired position measurements is required tocalculate those measurements. Correlated strain signals in excess of theminimum required set may be included in the position calculation andused to improve the signal to noise ration of the measurement. If astrain gage (320, 340) or sensor failure occurs the calculation may beadjusted to maintain position output albeit with a reduced signal tonoise ratio. In this way correlated signals provide redundancy as wellas an improved signal to noise ratio. Referring to FIG. 11F, a pivotmount including eight correlated strain sensors is illustrated inaccordance with an embodiment of the invention. Specifically, FIG. 11Fis an exemplary illustration similar to FIG. 8A described above,including 16 total strain sensing elements 320 (strain gages) and 16total reference gages 340. In such a configuration, a pair of strainsensing elements (strain gages) and references gauges on opposite sidesof a switch-back may correspond to a single strain sensor. As previouslydescribed, these pairs of strain sensing elements 320 on opposite sidesof a switch-back measure opposite strain types, of equal magnitude.Accordingly, in addition to the following discussion, these strain gages(as well as the corresponding reference gages 340) can also beconsidered correlated sensors. The strain sensors illustrated in FIG.11E may be linearly dependent sets (correlated pairs) depending uponwhether the pivot platform is rotated about the x-axis, rotated aboutthe y-axis, or is subjected to a vertical displacement. Table I belowdescribes certain correlated pairs of the exemplary embodiment.

TABLE I Correlated pair strain sensors Under rotation about the x axissignal 1 = signal 2 signal 3 = signal 4 signal 5 = −signal 6 signal 7 =−signal 8 Under rotation about the y-axis signal 1 = −signal 2 signal 3= −signal 4 signal 5 = signal 6 signal 7 = signal 8 Under verticaldisplacement signal 1 = signal 2 signal 3 = signal 4 signal 5 = signal 6signal 7 = signal 8

In the above exemplary embodiment, several correlated pairs aredescribed for an 8 channel (signal) operation, with each channelcorresponding to a signal produced by a pair of strain gages andreferences gages adjacent a switch-back. Under normal operation, thefeedback signal produced by the exemplary pivot mount operating undernormal operation can be converted into a synthesized output signal bytransformation matrix equation (1):

$\begin{matrix}{\begin{bmatrix}\theta_{x} \\\theta_{y} \\Z_{\;}\end{bmatrix} = {\begin{bmatrix}1 & 1 & {- 1} & {- 1} & 1 & {- 1} & 1 & {- 1} \\1 & {- 1} & 1 & {- 1} & 1 & 1 & {- 1} & {- 1} \\1 & 1 & 1 & 1 & 1 & 1 & 1 & 1\end{bmatrix}\begin{bmatrix}S_{1} \\S_{2} \\S_{3} \\S_{4} \\S_{5} \\S_{6} \\S_{7} \\S_{8}\end{bmatrix}}} & (1)\end{matrix}$

While embodiments of pivot mounts have been described thus far in asquare configuration, with switch-backs located along the x-direction ory-direction, embodiments are not so limited. Indeed, the strain sensingelements and reference gages can be located along a number ofdirections. A generalized transformation matrix for converting a pivotmount feedback signal to a synthesized output signal is represented inequation (2) for n strain signal inputs to 3 position measurementoutputs (e.g. tilt, tip, z):

$\begin{matrix}{\begin{bmatrix}{Out}_{1} \\{Out}_{2} \\{Out}_{3}\end{bmatrix} = {\begin{bmatrix}A_{1} & A_{2} & A_{3} & \ldots & A_{n\;} \\B_{1} & B_{2} & B_{3} & \ldots & B_{n} \\C_{1} & B_{2} & C_{3} & \ldots & C_{n}\end{bmatrix}\begin{bmatrix}S_{1} \\S_{2} \\S_{3} \\\ldots \\S_{n}\end{bmatrix}}} & (2)\end{matrix}$

In an embodiment illustrated in FIG. 12, a pivot mount 300 includes abase 302, pivot platform 304, and spring arms 306. Each spring arm 306is fixed to the pivot platform 304 at a corresponding inner root 350,and fixed to the base at a corresponding outer root 352. Each spring arm306 includes at least one switch-back along an axial length 354 of thespring arm such that a pair of lengths of the spring arm adjacent theswitch-back are parallel to each other. In the embodiment illustrated inFIG. 12, each spring arm 306 includes an inner switch-back 356 along aninner length of the spring arm and an outer switch-back 358 along anouter length of the spring arm. In an embodiment, an inner length 370 ofthe spring arm extending from the pivot platform 304 (along the axiallength 354 of the spring arm 306) is perpendicular to the inner root350. In an embodiment, an outer length 372 of the spring arm extendingfrom the base 302 (along the axial length 354 of the spring arm 306) isperpendicular to the outer root 352. The pivot mount illustrated in FIG.12 differs from other embodiments of pivot mounts described herein inthat the spring arms 306 are arranged in a generally equilateraltriangular configuration, rather than a generally square configuration.As a result, strain measured is not located within the only the εx andεy directions. Nevertheless, the same results of equal and oppositestrain, uniform bending moments in the high strain regions, anddistributed, correlated pairs is achieved. Accordingly, whileembodiments of the invention have been described specific to creatingand measuring strain in the εx and εy directions, embodiments are not solimited, and pivot mount feedback signals may be converted into asynthesized output signal for a variety of geometries.

In accordance with embodiments of the invention, the transfer headassembly 200 may adjust the orientation of the MPA 103 until a desiredamount of and/or a desired distribution of pressure across pivot mount300 is sensed by the pivot mount 300 strain sensing elements 320. Thus,the transfer head array 115 on MPA 103 may be actively aligned with anarray of micro devices on a mating substrate. For example, the spatialorientation representing alignment may be predetermined to include aplane passing through the transfer head array 115 being parallel to aplane passing through the array of micro devices. Alternatively, thespatial orientation representing alignment may include the planes notbeing parallel, but rather, being in some predetermined mutualorientation, such as angled such that only a portion of the transferhead array 115 make contact with respective micro devices when thearrays are brought together. More particularly, the spatial orientationrepresenting alignment of the transfer head array 115 with the array ofmicro devices may be any predetermined spatial orientation. Such spatialorientation may be monitored, sensed, and measured to determine systemcharacteristics such as distribution of pressure across pivot mount 300.Thus, the measured system characteristics may be used as a proxy torepresent alignment. Active alignment may increase the transfer rate ofmicro devices, since fine-alignment may be accomplished while pickingup, and similarly while releasing, the micro devices. Furthermore,active alignment may be made on-the-fly without parasitic translation ofthe transfer head array 115 that may otherwise smear and damage thearray of micro devices. Such on-the-fly adjustments may be useful when adonor substrate, e.g., carrier substrate, and/or a display substrate,e.g., receiving substrate, include surface irregularities and non-planarcontours.

Referring to FIG. 13A, a schematic illustration of a control scheme forregulating a transfer head assembly is shown in accordance with anembodiment. More particularly, the control loop may include multiplesub-loops that process a combination of position and strain inputs. Theactuators of transfer head assembly may be driven by the sub-loops,first toward an initial desired location, and if contact between MPA 103and a target substrate is sensed, then the initial desired location maybe modified to move MPA 103 toward a desired stress state, e.g., toevenly distribute pressure across MPA 103 and/or to achieve a desiredlevel of pressure at one or more locations on pivot mount 300 based on adeflection of the pivot mount 300 spring arms 306.

A primary input 1302 may define a set of reference signals thatcorrespond to an initial desired state of MPA 103. More specifically,primary input 1802 may define a target spatial location of MPA 103relative to an anticipated location of a micro device array or substratesurface. Primary input 1302 may be fed into one of several inner loops,each of which may correspond to an individual actuator. For example,x-actuator inner loop 1304 may correspond to a control loop forcontrolling an x-actuator of the transfer head assembly, and thus MPA103, to tip about a remote rotational center. Similarly, y-actuatorinner loop 1306 may correspond to a control loop for controlling ay-actuator of the transfer head assembly, and thus MPA 103, to tiltabout the remote rotational center. Also, z-actuator inner loop 1308 maycorrespond to a control loop for controlling a z-actuator of thetransfer head assembly and thus a location of MPA 103 along a z-axis.Therefore, the combination of inner loops allow for the control ofactuators that adjust a tip, tilt, and z-spatial orientation of MPA 103.

In an embodiment, inner loop control of transfer head assembly 200actuators results in a primary output 1310. More specifically, primaryoutput 1310 may be an instantaneous geometric configuration of transferhead assembly 200 resulting from actuator movement. The geometricconfiguration may be inferred from data supplied by encoders or othersensors that track spatial position of individual transfer head assembly200 components. That is, the geometric configuration may include acombination of individual geometric configurations such as a tipposition, tilt position, and z-position. Primary output 1310 may alsorelate to a spatial position of MPA 103 as inferred from known physicaldimensions of transfer head assembly 200 components. Alternatively, MPA103 surface location may be sensed directly using, e.g., lasermicrometers, accelerometers, etc., to provide spatial orientationfeedback that may be included directly in primary output 1310. Thus, aposition of MPA 103 may be inferred or sensed to determine whetherprimary output 1310 has been achieved, i.e., equals the intended primaryinput 1302. However, although MPA 103 may be driven toward a targetsubstrate to achieve the positional command of primary input 1302, insome cases, MPA 103 may contact the target substrate. Furthermore, oncecontact is detected, primary input 1302 may be modified by additionalcommands from several actuator outer loops, to achieve a neutral tip andtilt deformation of pivot mount 300 with a desired pressure distributionacross pivot mount 300. Accordingly, MPA array 103 may be driven to atip deflection, tilt deflection, and z-compression target within anaccuracy in the submicron range, e.g., on the order of less than about250 nm.

After contact between a transfer head array 115 of MPA 103 and a microdevice has been made, MPA 103 may be finely adjusted based on pressurefeedback from the pivot mount 300. More particularly, fine adjustment ofMPA 103 may be enabled in response to system recognition of a contactdisturbance 1312. In an embodiment, enable logic is included todetermine whether a contact disturbance 1312 is sensed prior to MPA 103achieving the desired primary input 1302, and if a contact disturbance1312 is sensed, additional control loops may be closed to permit fineadjustment of the transfer head assembly 200. More specifically,additional control loops may be closed to drive MPA 103 toward tipdeflection, tilt deflection, and z-compression targets, rather thantoward the initial positional target of primary input 1302.

In an embodiment, a contact disturbance 1312 is sensed when, e.g., MPA103 contacts a mating substrate out of alignment. For example, if MPA103 and the mating substrate make contact in perfect alignment, theprimary output 1810 may equal the primary input 1802 and micro devicesmay then be gripped by transfer head array 115 without requiringadditional adjustment. However, if MPA 103 and the mating substrate arenot perfectly aligned, displacement or strain measurements from eachstrain sensing element 320 on pivot mount 300 may be substantiallydifferent from each other and/or the desired level of pressure may notbe achieved. That is, in an embodiment, an expected or desired tip,tilt, and compression state must be satisfied prior to initiatingelectrostatic gripping. If the desired state is not achieved,displacement or strain measurements may be fed as feedback signals 1314.

In an embodiment, feedback signals 1314 correspond to analog signalsfrom the strain sensing elements 320 and references gages 340. In theexemplary embodiment above, feedback signals 1314 may include eightsensor signals from sixteen separate strain sensing elements 320 andsixteen reference gages 340. The feedback signals 1314 may beconditioned by a signal conditioning and combination logic 1315 totransform the analog signals into a synthesized output signalrepresenting a strain state of a respective strain sensing element.These synthesized output signals may furthermore be combined by signalconditioning and combination logic 1315 to synthesize one or more of apivot mount 300 compression synthesized output signal, a pivot mount 300tilt deflection synthesized output signal, and a pivot mount 300 tipdeflection synthesized output signal represented by a transformationmatrix equation, such as equation (1) or equation (2) described above .The synthesized output signals may be provided as inputs to dynamiccontrol enable logic 1316. More particularly, dynamic control enablelogic 1316 may observe the one or more synthesized output signals todetermine that a contact disturbance 1312 has occurred in one or more ofa tip, tilt, or z-direction. For example, if a non-zero compressionsignal is synthesized by signal conditioning and combination logic 1315that exceeds predetermined limits, dynamic control enable logic 1316 mayrecognize the contact disturbance 1312.

In response to observing that a contact disturbance 1312 exists, dynamiccontrol enable logic 1316 may close respective outer loops, each ofwhich may be configured to provide output commands to modify thepositional command of primary input 1302. Thus, closing the outer loopsmay drive the actuators to achieve a desired state of pressure andorientation, rather than driving them to achieve an initial positioncommand. For example, if dynamic control enable logic 1316 observes thata compression contact disturbance 1312 exists, z-actuator outer loop1318 may be closed to respond to the contact disturbance 1312 byadjusting a z-actuator. Likewise, dynamic control enable logic 1316 mayrespond to tip deflection signals or tilt deflection signals by enablingx-actuator outer loop 1320 or y-actuator outer loop 1322, respectively.

Deflection and compression feedback signals may be passed from signalconditioning and combination logic 1315 as synthesized output signals torespective outer loops for comparison with deflection command inputs1340 provided to respective outer loops. In an embodiment, pivot mount300 deflection command inputs 1340 may correspond to a desired pressuredistribution across pivot mount 300 or MPA 103. Thus, pivot mount 300deflection command inputs 1340 may represent tip deflection, tiltdeflection, and z-compression targets of pivot mount 300. These targetsmay be compared to the synthesized output signals from signalconditioning and combination logic 1315, which indicate an instantaneouspressure distribution across pivot mount 300, to determine a difference.The difference, if any, may then be fed as an error signal to driverespective transfer head assembly 200 actuators. For example, if tippingof pivot mount 300 is sensed as a contact disturbance 1312 and dynamiccontrol enable logic 1316 observes that the tipping exceeds an allowableamount, x-actuator outer loop 1320 may be closed and the tippingdeflection signal may be compared with a pivot mount 300 tip deflectioncommand 1340 to generate a motion control signal that will tip pivotmount 300 toward a desired stress state. The motion control signal maybe fed to a servo filter and passed through inverse kinematicscalculations to generate an outer loop command output 1330. In anembodiment, the motion control signal may also be added with othertransfer head assembly motion control signals at one or more of motionsummation nodes 1350. This may be the case, for example, when movementof multiple actuators is required to cause tipping.

In order to close the control loop, the outer loop command outputs 1330may be combined with primary input 1302 and passed back into actuatorinner loops. For example, a tipping outer loop command 1330 may besummed with primary input 1302 for an x-actuator and passed throughx-actuator inner loop 1304, thereby controlling an x-actuator in such amanner that pivot mount 300 tips toward a physical state of more evenpressure distribution. Respective outer loop commands may be passedthrough to any actuator inner loop for which a contact disturbance 1312was sensed.

The above control methodology may be performed and repeated until thetransfer head assembly 200 is moved to a location at which pressuredistribution across pivot mount 300, and hence MPA 103, is uniform andachieves a desired amount of pressure. Thus, transfer head assembly 200may be controlled to bring an array of electrostatic transfer head array115 on MPA 103 into contact with an array of micro devices on a matingsubstrate. Using the control system described above, if alignmentbetween MPA 103 and the mating substrate is not initially perfect, whichwould be true of almost every transfer operation, pressure distributioncontrol may be implemented to fine tune the alignment. The controlmethodology may be performed quickly, e.g., on the order of about 50 msto sense a contact disturbance 1312, enable the appropriate outerloop(s), and feed appropriate outer loop control commands to actuators,and thus, complete contact may be rapidly achieved between anelectrostatic transfer head array 115 and an array of micro devices,enabling efficient transfer between a carrier substrate and a receivingsubstrate.

Referring now to FIG. 13B, a schematic illustration is provided for amethod of generating a synthesized output signal in an embodiment. Asillustrated, feedback signals 1314 are received by a signal conditioningand combination logic 1315, which combines the incoming feedback signals1314 from the pivot mount 300 and generates synthesized output signals.In the simplest case, feedback signals received from the pivot mount(e.g. from sensors 1-8 described above with regard to FIG. 11F) arelinearly combined by multiplication with a transformation matrix to forma set of output measurements (synthesized output signals).

Referring to the embodiment illustrated in FIG. 13C, in a more compleximplementation, correlated sets of strain sensors may be checked forsignal quality. As illustrated, feedback signals 1314 are received by asignal conditioning and combination logic 1315. At 1315A, the feedbacksignals 1314 are checked to determine if they are within a predefinednormal operating range. Sensors (including gages 320, 340) that areoutside of the normal operating range are flagged as failed sensors.Failed sensor signals may then be rejected requiring a change in thetransformation matrix. At 1315B signals are checked for variation withinthe normal operating range. Sensors (including gages 320, 340) withvariation that is greater or less than a normally operating sensor areflagged as failed sensors. Based on the sensors flagged as failed, atransformation matrix is selected that is able to synthesize the outputsfrom the remaining signals, and the transformation matrix is used toconvert the resulting sensor signal vector into synthesized outputsignals (position measurement output) at 1315C. In this way synthesizedoutput signals are maintained at a reduced signal to noise ratio ratherthan sensor failure causing output failure.

Examples of generating a synthesized output signal utilizing the 8channel embodiment of FIG. 11F and the transformation matrix equation(1) are provided in FIGS. 13D-13E. It is to be appreciated that thefollowing examples are provided for illustrational purposes, and thatembodiments are not limited to the particular geometries or number ofchannels in the exemplary embodiments. Referring to FIG. 13D, at 1315Asignal 2 is determined to read low (outside of the normal operationrange) by the signal conditioning and combination logic 1315, and isflagged as a failed sensor. All remaining sensors are determined to beoperating within normal variation in the operating range at 1315B. At1315C, based on sensor 2 as being flagged as failed, a transformationmatrix is selected and used to convert the resulting sensor signalvector into synthesized output signals (position measurement output).Referring to FIG. 13D, at 1315A signal 2 is determined to read low(outside of the normal operation range) and signal 5 is determined toread high (outside of the normal operation range) by the signalconditioning and combination logic 1315, and are flagged as failedsensors. At 1315B, signal 7 is determined to have variation that islower than normal variation by the signal conditioning and combinationlogic 1315, and is flagged as a failed sensor. At 1315C, based onsensors 2, 5, and 7 as being flagged as failed, a transformation matrixis selected and used to convert the resulting sensor signal vector intosynthesized output signals (position measurement output).

Referring to FIG. 14, a flowchart illustrating a method of aligning aMPA 103 coupled with a pivot mount 300 on a transfer head assembly 200relative to a target substrate is shown in accordance with anembodiment. The method may be performed, e.g., during a pick-up or aplacement operation as micro devices are transferred from a carriersubstrate to a receiving substrate. At operation 1402, mass transfertool 100 moves transfer head assembly 200 along a z-axis toward a targetsubstrate, e.g., carrier substrate held by carrier substrate holder 104or receiving substrate held by receiving substrate holder 106, accordingto primary input 1302. More specifically, the MPA 103 and pivot mount300 are moved toward the target substrate along the z-axis. Movement ofMPA 103 along z-axis 510 may be achieved by actuating various actuatorsof mass transfer tool 100 or a substrate holder.

Initially, there may be no compressive loading applied to MPA 103 orpivot mount 300. This initial state may correspond to a range of travelover which array of micro devices are physically separated from theelectrostatic transfer head array. During this travel, MPA 103 and thetarget substrate may have misaligned surfaces, but there may be noindication of this misalignment since the pressure distribution state ofpivot mount 300 may be uniform, i.e., all strain sensing elements may beoutputting signals indicating zero strain.

At operations 1404 and 1406, an electrostatic transfer head in theelectrostatic transfer head array 103 may contact a micro device whileother electrostatic transfer heads may remain separated fromcorresponding micro devices. That is, contact may be made while MPA 103is misaligned with the target substrate. This positional misalignmentmay be sensed as uneven pressure distribution in pivot mount 300. Forexample, a first strain output value from one strain sensing element 320on pivot mount 300 and a different second strain output value fromanother strain sensing element 320 in pivot mount 300 may differ. Thestrain signals may be provided as feedback signals 1314 and conditionedand combined by into synthesized output signals (e.g. tip deflection,tilt deflection, and compression signals) by signal conditioning andcombination logic 1315 indicating a contact disturbance 1312.

Dynamic enable control logic 1316 may observe that the contactdisturbance 1312 exists, and depending upon the level of contactdisturbance 1312, may activate actuator outer loops to determine drivingsignals for actuating various actuators of transfer head assembly 200 inorder to adjust an orientation of MPA 103 such that pressuredistribution across pivot mount 300 is uniform. For example, atoperation 1408, in response to the tip signal being recognized as acontact disturbance 1312 above a threshold, x-actuator outer loop 1320may feed command signals 1330 to x-actuator inner loop 1304 in order toactuate an x-actuator to tip MPA 103 about remote rotational center.Similarly, at operation 1410, in response to the tilt deflection signalbeing recognized as a contact disturbance 1312 above a threshold,y-actuator outer loop 1322 may feed command signals to y-actuator innerloop 1306 in order to actuate a y-actuator 708 to tile MPA 103 aboutremote rotational center.

At operation 1412, in response to actuation of the x- and y-actuatorsbased on the tip and tilt deflection signals MPA 103 may be rotated intoalignment with the target substrate. Furthermore, with remote rotationalcenter co-located with the contact surface of MPA 103, the electrostatictransfer head array 115 may experience pure rotation about remoterotational center. Thus, as MPA 103 is aligned with the targetsubstrate, the electrostatic transfer head array 115 may experienceminimal parasitic lateral motion and micro devices may remain undamaged.

Actuation of transfer head assembly 200 according to synthesized outputsignals (tip, tilt, and z-compression signals) may continue until theelectrostatic transfer head array 115 is in contact with micro deviceson the target substrate. More particularly, actuation may continue untilprimary output 1310 is within the limits set by primary input 1302, atwhich point actuation may be stopped. As discussed above, primary output1310 may be a positional output that is modified to reach a desiredpivot mount 300 state. For example, actuation of transfer head assembly200 may continue until primary positional input is achieved and/orpressure distribution across pivot mount 300 is uniform.

After contact between the electrostatic transfer head array 115 and themicro devices is made, a voltage may be applied to the electrostatictransfer head array 115 to create a grip pressure on the array of microdevices. An electrostatic voltage may be applied to electrostatictransfer head array 115 compliant voltage contacts 316 and voltagecontacts 120. Additional electrical contacts and connectors may beintegrated within transfer head assembly 200 and powered by voltagesupplies based on control signals from computer 108. For example,computer 108 may implement a control algorithm instructing thatelectrostatic transfer head array 115 be activated if a predefineddeformation is simultaneously sensed by each displacement sensor onpivot mount 300 during a pick up process. As a result, the array ofelectrostatic transfer head array 115 may apply a gripping pressure tothe array of micro devices after the entire array surface is in contactand uniform pressure is applied across the array.

After gripping the micro devices with electrostatic transfer head array115, the micro devices may be picked up from carrier substrate. Duringpick up, the electrostatic voltage supplied to the electrostatictransfer head array 115 may persist, and thus, the array of microdevices may be retained on the electrostatic transfer head array 115 andremoved from the carrier substrate.

During the pick up operation, a heating element may direct heat towardpivot mount 300 and/or MPA 103. Thus, the micro devices may be heatedthrough contact with electrostatic transfer head array 115 on M PA 103during pick up. For example, a heating element adjacent to pivot mount300 may be resistively heated to transfer heat to MPA 103, and thus, tothe micro devices through the electrostatic transfer head array 115.Heat transfer may occur before, during, and after picking up the arrayof micro devices from carrier substrate.

Although a pick up process is described in relation to FIG. 14, asimilar methodology may be used to control the placement of microdevices onto a receiving substrate, such as a display substrate, held byreceiving substrate holder 106. For example, as the micro devices aregripped by the electrostatic transfer head array 115, mass transfer tool100 may move the MPA 103 over a receiving substrate, and align MPA witha target region of the receiving substrate. MPA 103 may be advancedtoward, and aligned with, the receiving substrate using the controlsequence described above until the array of micro devices held by theelectrostatic transfer head array 115 are placed in uniform contact withthe target region. Uniform contact may be inferred by sensing a strainstate of pivot mount 300. Subsequently, voltage may be removed from theelectrostatic transfer head array 115 to release the micro devices ontothe receiving substrate and complete the transfer operation.

Referring to FIG. 15, a schematic illustration of a computer system isshown that may be used in accordance with an embodiment. Portions ofembodiments of the invention are comprised of or controlled bynon-transitory machine-readable and machine-executable instructions thatreside, for example, in machine-usable media of a computer 108. Computer108 is exemplary, and embodiments of the invention may operate on orwithin, or be controlled by a number of different computer systemsincluding general purpose networked computer systems, embedded computersystems, routers, switches, server devices, client devices, variousintermediate devices/nodes, stand-alone computer systems, and the like.Furthermore, although some components of a control system, e.g., signalconditioning and combination logic 1315 and dynamic control enable logic1316, have been broken out for discussion separately above, computer 108may integrate those components directly or include additional componentsthat fulfill similar functions.

Computer 108 of FIG. 15 includes an address/data bus 1502 forcommunicating information, and a central processor 1504 coupled to bus1502 for processing information and instructions. Computer 108 alsoincludes data storage features such as a computer usable volatilememory, e.g. random access memory (RAM) 1506, coupled to bus 1502 forstoring information and instructions for central processor 1504,computer usable non-volatile memory 1508, e.g. read only memory (ROM),coupled to bus 1502 for storing static information and instructions forthe central processor 1504, and a data storage device 1510 (e.g., amagnetic or optical disk and disk drive) coupled to bus 1502 for storinginformation and instructions. Computer 108 of the present embodimentalso includes an optional alphanumeric input device 1512 includingalphanumeric and function keys coupled to bus 1502 for communicatinginformation and command selections to central processor 1504. Computer108 also optionally includes an optional cursor control 1514 devicecoupled to bus 1502 for communicating user input information and commandselections to central processor 1504. Computer 108 of the presentembodiment also includes an optional display device 1516 coupled to bus1502 for displaying information.

The data storage device 1510 may include a non-transitorymachine-readable storage medium 1518 on which is stored one or more setsof instructions (e.g. software 1520) embodying any one or more of themethodologies or operations described herein. For example, software 1520may include instructions, which when executed by processor 1504, causecomputer 108 to control mass transfer tool 100 or remote center robot500 according to the control scheme described above for aligning an MPA103 with a target substrate. Software 1520 may also reside, completelyor at least partially, within the volatile memory, non-volatile memory1508, and/or within processor 1504 during execution thereof by computer108, volatile memory 1506, non-volatile memory 1508, and processor 1504also constituting non-transitory machine-readable storage media.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming a pivot mount withintegrated strain sensing elements and/or compliant voltage contacts.Although the present invention has been described in language specificto structural features and/or methodological acts, it is to beunderstood that the invention defined in the appended claims is notnecessarily limited to the specific features or acts described. Thespecific features and acts disclosed are instead to be understood asparticularly graceful implementations of the claimed invention usefulfor illustrating the present invention.

What is claimed is:
 1. A pivot mount comprising: a pivot platform; abase; a plurality of spring arms, each spring arm fixed to the pivotplatform at a corresponding inner root, and fixed to the base at acorresponding outer root; wherein each spring arm includes: aswitch-back along an axial length of the spring arm such that a pair offirst and second lengths of the spring arm immediately adjacent theswitch-back are parallel to each other; a first strain sensing elementat the first length; and a second strain sensing element at the secondlength.
 2. The pivot mount of claim 1, wherein the switch-back is aninner switch-back along an inner length of the spring arm, and furthercomprising an outer switch-back along an outer length of the spring arm.3. The pivot mount of claim 1, further comprising a first reference gageadjacent the first strain sensing element at the first length.
 4. Thepivot mount of claim 3, further comprising a second reference gageadjacent the second strain sensing element at the second length.
 5. Thepivot mount of claim 1, wherein the plurality of spring arms comprisesthree or more spring arms.
 6. The pivot mount of claim 5, wherein theswitch-back is an inner switch-back along an inner length of the springarm, and further comprising an outer switch-back along an outer lengthof the spring arm.
 7. The pivot mount of claim 1, wherein the inner rootis perpendicular to an inner length of the spring arm extending from thepivot platform, and the outer root is perpendicular to an outer lengthof the spring arm extending from the base.
 8. The pivot mount of claim1, wherein the pivot platform is movable relative to the base in adirection orthogonal to a contact surface of the pivot platform, whereinmovement of the pivot platform in the direction orthogonal to thecontact surface of the pivot platform causes a normal strain that ischaracterized as being parallel to the axial length of the spring arm atthe first and second lengths of the spring arm.
 9. The pivot mount ofclaim 8, wherein the normal strain comprises opposite sign on the firstand second lengths of the spring arm.
 10. The pivot mount of claim 1,wherein the first and second strain sensing elements are strain gages.11. The pivot mount of claim 10, wherein the strain gages are bonded tothe spring arm, deposited on the spring arm, or doped regions in thespring arm.
 12. A pivot mount comprising: a pivot platform including aplurality of compliant voltage contacts; a base; and a plurality ofspring arms, each spring arm fixed to the pivot platform at acorresponding inner root, and fixed to the base at a corresponding outerroot.
 13. The pivot mount of claim 12, wherein each compliant voltagecontact comprises a channel extending through the pivot platform. 14.The pivot mount of claim 13, wherein the channel for each compliantvoltage contact is characterized as a winding contour or switch-back.15. The pivot mount of claim 13, further comprising a clamping electrodeon the pivot platform.
 16. The pivot mount of claim 15, wherein eachcompliant voltage contact protrudes from the pivot platform such thatthey are elevated above the pivot platform and the clamping electrode.17. A transfer tool comprising: an articulating transfer head assembly;a pivot mount, mountable onto the articulating transfer head assemblycomprising: a pivot platform; a base; a plurality of spring arms, eachspring arm fixed to the pivot platform at a corresponding inner root,and fixed to the base at a corresponding outer root; wherein each springarm includes: a switch-back along an axial length of the spring arm suchthat a pair of first and second lengths of the spring arm immediatelyadjacent the switch-back are parallel to each other; a first strainsensing element at the first length; a second strain sensing element atthe second length; and a micro pick up array mountable onto the pivotplatform of the pivot mount, the micro pick up array including an arrayof electrostatic transfer heads.
 18. The transfer tool of claim 17,wherein the pivot platform comprises a plurality of compliant voltagecontacts.
 19. The transfer tool of claim 18, wherein the micro pick uparray comprises a plurality of voltage contacts arranged to mate withthe plurality of compliant voltage contacts of the pivot platform. 20.The transfer tool of claim 17, wherein each electrostatic transfer headhas a localized contact point characterized by a maximum dimension of1-100 μm in both x- and y-dimensions.